JHUG Meetup

February 2018

A short introduction

to the Kotlin language

for Java developers

Antonis Lilis

Mobile Engineer @

Some History

● 2011: JetBrains unveiled Project Kotlin, a new language for the JVM

● 2012: JetBrains open sourced the project under the Apache 2 license

● 2016: Kotlin v1.0 is released

● 2017: Google announced first-class support for Kotlin on Android

● Kotlin is technically 7, but in reality 2 years old

Trivia: The name comes from a small island in the Baltic Sea, near St.Petersburg. The language team decided to name it after an island just like Java (though Java was perhaps named after the coffee)

The Kotlin Language

● Statically Typed

○ Type validation at compile time

● Supports Type Inference

○ Type automatically determined from the context

● Both Object Oriented and Functional

● First-class functions

○ You can store them in variables, pass them as parameters, or return them

from other functions

● Was designed with Java Interoperability in mind

Constants and Variables

Control Flow

● Classic loops:

○ if

○ for

○ while / do-while

● when

○ Replaces the switch operator

○ No breaks, no errors

Functions

● Named arguments

● Can be declared at the top level of a file (without belonging to a class)

● Can be Nested

● Can have a block or expression body

Functions

● Default parameter values

○ Avoids method overloading and boilerplate code

Simple string Interpolation

Classes

"Any" is the

analogue of java

Object: a superclass

of all classes

data classes:

autogenerated

implementations of universal methods (equals, hashCode

etc)

Properties

● first-class language feature

● combination of the field

and its accessors

Modifiers

No static keyword

● Top-level functions and properties

(e.g. for utility classes)

● Companion objects ● The object keyword:

declaring a class and creating an instance

combined (Singleton)

Extensions

● Enable adding methods and properties to other people's classes

○ Of Course without access to private or protected members of the class

Null Checks

● Safe-call operator ?.

● Elvis operator ?:

● The let function

"I call it my billion-dollar mistake.

It was the invention of the null reference in 1965"

Tony Hoare

Not-null assertion operator !!

Safe Casting

● Safe cast operator as?

● Smart cast

○ combining type checks

and casts

Collections

● Kotlin enhances the Java collection classes (List, Set, Map)

Chained

Calls

Delegation

● Composition over Inheritance

design pattern

● Native support for delegation

(implicit delegation)

● Zero Boilerplate code

● Supports both Class Delegation and

Delegated Properties

Class Car inherits from an interface Nameable

and delegates all of its public methods to a

delegate object defined with the by keyword

Lamdas and Higher Order Functions

Domain-specific language construction

● Kotlin provides mechanisms for creating internal DSLs that use exactly the

same syntax as all language features and are fully statically typed

Coroutines

● Introduced in Kotlin 1.1 (March 2017)

● A way to write asynchronous code sequentially

● Multithreading in a way that is easily debuggable and maintainable

● Based on the idea of suspending function execution

● More lightweight and efficient than threads

Source Code Layout

● Packages (similar to that in Java)

● Multiple classes can fit in the same file

● You can choose any name for files (not restricted to class name)

● The import keyword is not restricted to importing classes

○ Top-level functions and properties can be imported

● Kotlin does not impose any restrictions on the layout of source files on disk

● Good practice to follow Java's directory layout

○ Especially if mixed with java

Kotlin IDEs

● You can write Kotlin next to Java in

your favorite IDE

● Kotlin works with JDK 1.6+

● IntelliJ IDEA (and Android Studio)

support Kotlin out of the Box

● There is a plugin for Eclipse too

Hello World

● Kotlin files have .kt extension

● You can also try your code in

REPL (Read-Eval-Print-Loop)

Let's Mix with some Java

Java from Kotlin

● You can call Java code from you Kotlin files transparently

Convert Java to Kotlin

Code size

● According to Jetbrains converting a Java application to Kotlin is expected

to reduce the line count by 40%

○ More concise language (eg. Kotlin data classes can replace 50 line

classes with a single line)

○ The Kotlin standard library enhances existing Java classes with

extensions that trivialize common usage (eg. collections)

○ Kotlin allows you to extract more re-usable patterns than what Java

allows

Utility

● Top-level computed property

● String extension

Kotlin from Java

● In most cases the

integration is Seamless

● Some Kotlin features do

not exist in Java (e.g. top

level functions or

properties)

● In such cases conventions

are used

● In our case a static class is

generated for the top-level

declarations

Libraries & Resources

● You can use Any Java Library since Java and Kotlin are 100% interoperable

● Kotlin libraries: a nice curated list at https://kotlin.link

● Kotlin popularity is growing and resources become more abundant

Kotlin also lives outside the JVM

● Kotlin 1.1 (March 2017): officially

released the JavaScript target,

allowing you to compile Kotlin code

to JS and to run it in your browser

● Kotlin 1.2 (November 2017): adding

the possibility to reuse code

between the JVM and JavaScript

● Kotlin/Native v0.6 (Valentine's Day

release 2018): Better support for

native targets (e.g. iOS,

WebAssembly, Windows)

Any Disadvantages?

● An app built with Kotlin will likely result in a larger file package size than one

built purely in Java

● The build time for Kotlin is a little slower

Final Thoughts  I am HERE!

[REF: Pinterest Engineering]

● The learning curve

○ IMHO comparatively small

● Not so popular yet

○ 44th in TIOBE Index for

February but competed

with C for language of the

year 2017

● Development Stability

○ Tools still in Beta

○ Static Analysis Tools

● Reversibility

○ Once you Go Kotlin

IMHO Kotlin is here to stay

Thank you!

Questions?

http://antonis.me/